1. Field of the Invention
The invention relates to coated abrasives and specifically to coated abrasive products. More in particular, the invention relates to coated abrasive products and methods of making the products, and to the use of resin coatings on webs of cloth or paper to produce an article of commerce which may be subsequently useful to produce coated abrasive materials.
2. Prior Practices
It is well known in the coated abrasive art to apply a resinous binder and abrasive grains to a paper or cloth substrate and cure the binder to produce a coated abrasive product. Typically an overcoat is applied over the grain carrying layer and, in turn, cured. Examples of suitable binders are glutelin glue and phenolic resins. If water-proof products are desired, polyurethane resins, epoxy resins and alkyd resins, possibly in combination with melamine resins are typically used. Special techniques and apparatus are necessary for the lengthy curing process. To avoid destruction of the fibrous substrates, usually consisting of polyester or cellulose, curing is carried out at a maximum temperature of 120.degree. to 130.degree. C. Rapid curing which would allow the use of horizontal dryer, is not generally employed because gas bubbles form in the resin which adversely affect the adhesion of the resin on the substrate. Curing of the coated material usually requires one or more hours, and is therefore conveniently carried out in a loop dryer. The loop dryers through which the coated web material is passed, enable a long curing process, but there are also disadvantages, such as defects which may be formed in the areas of the material where it is suspended, sagging of the binder, and changing of the grain position due to the vertical suspension. Also variations in temperature in the loop dryer produced by the necessary slow air circulation can cause inconsistent or incomplete drying or curing of the binder material.
In addition, there are several other disadvantages in the present commercial practice of forming coated abrasives. For example, numerous separate curing steps are required in the typical process for the production of cloth or paper-backed abrasives. Generally, the substrate is initially treated to prepare a base for application of abrasives. Subsequently, coated abrasive materials are produced using the base. The base, e.g., cloth, is usually coated with at least one back coat of resin which impregnates the cloth with resin and fills the interstices in the back of the cloth. The base cloth is also typically coated with at least one face coat to fill interstices of the cloth on the face side. Subsequently, a make coat is applied over the base coat, the abrasive grain added and a size coat or an overcoat, applied.
As discussed above the drying, or curing, of the various coats is generally carried out in a loop dryer. Such driers require a substantial amount of floor space and have high energy requirements. In addition, the temperature in large dryers is difficult, at best, to control. Further, there is a problem of resin and grain shifting positions during curing because of the long hang times required.
It has been suggested in U.S. Pat. No. 4,047,903, Hease et al, that waterproof abrasives be produced using resins cured by an electron beam. However, there has remained a need for viable methods, formulations, and apparatus which would allow the commercialization of coated abrasive products produced by electron beam curing.
In the production of conventionally coated abrasive products, relatively thick coats of resin are utilized, as compared with the thickness generally used in electron beam curing. The relatively thick coats create handling problems as the resins do not rapidly cure and tend to remain sticky for a substantial period of time prior to being completely cured.
The present invention provides for the use of combinations of electron radiation curable resin systems and conventional heat curable resin systems. The combinations provide many of the advantages of systems employing solely electron radiation curable resins while utilizing more economical mixtures which include conventional heat curable resins. The hybrid mixtures of the present invention in which an electron radiation curable resin system and a conventional heat curable resin system are combined in a single coat, are particularly useful. Such mixtures are applied to a substrate and processed to cure the electron radiation curable resin portion. The resultant product is a combination of cured electron radiation curable resin and partially cured heat curable resin. The product, typically slightly tacky, is sufficiently nonadherent that it may be rolled and stored, or may be used for continued processing, without completely curing the heat curable resin component. Various layers of hybrid mixtures may be successively applied without completely curing the heat curable portions of the layers. The incompletely cured heat curable resins suitably are cured in a single, typically final, heat curing step.
The present invention provides a good quality coated abrasive product in a low-cost, commercially satisfactory manner.